This invention relates to an apparatus for detecting a focusing condition of an objective lens provided in an optical machinery such as a still camera comprising a focusing screen arranged at a position optically conjugated with a predetermined focal plane such as a film plane, a view finder for inspecting an image of an object formed on the focusing screen, at least one photodetector array arranged on or near a plane optically conjugated with the predetermined focal plane, means for projecting a part of the object image upon the photodetector array, and a control unit for processing output image signals supplied from the photodetector array in accordance with a given evaluation function to derive a signal representing the focusing condition of the objective lens.
Such a focus detecting apparatus is known and is used in a still camera. The apparatus comprises the photodetector array comprising a number of photodetectors. A part of an optical image of an object to be photographed is projected upon the photodetector array in such a manner that when an infocused image is projected upon a film, an infocused image is also projected upon the photodetector array. Therefore, by detecting a contrast or sharpness of the image projected upon the photodetector array, it is possible to detect the focusing condition of the objective lens. Usually the image detected by the photodetector array is selected to a central portion in the field of view and thus, when the camera is directed toward the object in such a manner that a subject in a scene is situated at the center of the field of view, the focusing condition of the objective lens with respect to the interesting subject can be correctly detected. However, the subject image is not always situated at the center of the field of view or an image of object in foreground or background may be projected upon the photodetector array. In such a case, the focusing condition of the objective lens could not be detected accurately and the objective lens might be erroneously focused onto the object in the foreground or background.